Management Scenario for the Critical Subwatersheds of Small Agricultural Watershed using SWAT model and GIS Technique Authors: M.P. Tripathi, N. Agrawal and M.K. Verma Indira Gandhi Krishi Vishwavidyalaya Raipur (C.G.) India and National Institute of Technology Raipur (C.G.) India
Objectives To calibrate and validate the physically based hydrological and water quality simulation model for the small watershed. To estimate surface runoff, sediment yield and nutrient losses from a small watershed using SWAT model, satellite data and GIS. To identify the critical sub-watersheds on the basis of estimated sediment yield and nutrient losses for multiple years. To develop and recommend the best management practices for the critical sub-watersheds of the study watershed.
CHHATTISGARH IN INDIA N Chhokranala Watershed Outlet Longitude: 81.70 0 to 81.76 0 E Latitude: 21.21 0 to 21.23 0 N Geographical area-1731 ha Location map of the Chhokranala Watershed
Salient Features of Chhokranala Watershed Altitude: 290 m to 310 m above MSL Area of watershed: 1731 Ha No of villages covered: 6 Weighted average slope : 1.6 % Predominant soil: Sandy clay loam Avg. annual rainfall: 1400 mm Mean monthly temp: Max. 38 0 C Mean monthly temp: Min. 7 0 C Mean RH: Min. 38 % Max. 83 % Over all climate: Sub-humid tropic
RSOP at the outlet of Chhokranala watershed
DEM SWAT Arc GIS Interface Output ERDAS Arc GIS Time series Observed runoff Simulated runoff Precipitation Data base Runoff (mm) 40 35 30 25 20 15 10 5 0 Jun-98 Jul-98 Aug-98 Sep-98 Oct-98 Month 0 25 50 75 100 125 150 175 200 Precipitation (mm) SWAT GIS Interface
Contour map of the Chhokranala watershed
Digital Elevation Model (DEM) of the watershed
Sub watershed map of the watershed
LAND USE/LAND COVER CLASSIFICATION The path 21, row 55, scenes of IRS-1C (LISS-III) satellite with date of pass 5th October 2002 is used for the study to prepare the land use/land cover maps of both the watersheds. Supervised classification, has been adopted for classifying the land use of the study watershed. This is an interactive approach in which the operator classifies an area or group of pixels that belong to one or more categories of specific land use/land cover. Variouslandusesaregivendifferentcoloursforeasy identification.
Land use/cover map of the watershed
Area under different land use in the Chhokranala WS Land use class No. of pixels Area (ha) % Image Water body 591 8.81 0.51 Grasses & shrubs 177 2.64 0.15 Orchard 254 3.79 0.22 Crop land 51605 769.49 44.45 Settlement 7837 116.87 6.75 Barren land 10183 151.84 8.77 Fallow land 45438 677.56 39.15 Total 116085 1731.00 100.00
Soil texture map of the watershed
Soil texture prevailing in the Chhokranala WS Soil type Local name Area (ha) % area Alfisols (Loam) Dorsa 231.23 13.36 Inceptisols (Sandy clay loam) Matasi 655.50 37.87 Vertisols (Clay) Kanhar 640.39 36.99 Entisols (Sandy loam) Bhata 203.88 11.78
Physical and chemical properties of soil Particulars Bhata Matasi Dorsa Khanar Soil depth (cm) 5-25 25-75 75-100 >100 Sand (%) 50-80 30-50 25-35 15-25 Silt (%) 15-25 30-50 25-30 20-30 Clay (%) 10-20 20-35 34-45 45-47 Bulk density (g/cc) 1.76-1.85 1.55-1.60 1.35-1.65 1.28-1.30 Infiltration (cm/hr) 4.0-6.0 0.6-2.9 2.0-3.0 1.0-2.5 Available water (cm) 2.15 8.67 17.5 16.8 ph 5.7-6.5 6.5-7.0 7.0-7.4 7.4-7.6
Subwatershed Sub-watershed wise values of various input parameters for the SWAT model Area (ha) Slope (%) Curve Numbers Av. slope length (m) Channel length (km) Channel slope (%) K value P value WS1 185.45 1.2 79.32 140.3 2.13.001 0.18 0.60 WS2 290.71 1.6 88.16 143.8 3.75.003 0.20 0.50 WS3 119.71 1.4 87.37 142.6 3.70.002 0.14 0.50 WS4 316.71 1.3 89.52 145.4 3.15.002 0.15 0.50 WS5 277.74 2.0 81.63 149.8 2.13.005 0.21 0.60 WS6 280.71 1.8 89.23 142.3 3.70.004 0.23 0.60 WS7 259.97 1.7 83.31 135.0 3.75.003 0.21 0.50 WS 1731.00 1.6 85.02 146.7 6.10.005 0.20 0.50
Calibrated parameters of the model for the watershed S. No. Calibrated parameters Values chosen Prescribed range 1 Base flow factor 0.50 0.0-1.0 2 Effective HC 25.0 6.5-25.0 3 Channel 'n' value 0.025 0.02-0.07 4 Overland flow 'n' value 0.040 0.03-0.09 5 Fraction of field capacity 0.0 0.0-1.0
200 180 160 140 Rainfall Observed Simulated 0 20 40 60 Runoff (mm) 120 100 80 60 40 20 0 6/1 7/1 8/1 9/1 10/1 Time (days) 80 100 120 140 160 180 200 Rainfall (mm) Graphical comparison of observed and simulated daily runoff (Calibration years 2003)
Regression line 1:1 line Simulated runoff (mm) 100 90 80 70 60 50 40 30 20 10 0 y = 1.0129x + 0.2775 r 2 = 0.9673 0 10 20 30 40 50 60 70 80 90 100 Observed runoff (mm) Scattergram between observed and simulated daily runoff (Calibration years 2003)
Results of statistical analysis for model calibration Daily (2003) Statistics Runoff (mm) Sediment yield (t/ha) Observed Simulated Observed Simulated Mean 5.46 5.82 0.173 0.190 Standard deviation 13.78 14.20 0.387 0.458 Maximum peak 86.67 84.47 2.1 2.3 Total 835.8 889.1 26.49 29.09 Count 153 153 153 153 t-calculated -1.670-1.160 t-critical (two tailed) 1.975 1.975 r 2 0.967 0.930 % deviation -6.373-9.815 COE 0.964 0.917
Rainfall Observed Simulated 7 0 6 20 40 Sediment yield (t/ha) 5 4 3 2 60 80 100 120 140 Rainfall (mm) Graphical comparison of observed and simulated daily sediment yields (Calibration years 2003) - 160 1 180 0 200 6/1 7/1 8/1 9/1 10/1 Time (days) Regression line 1:1 line 3 y = 1.2241x - 0.0218 Scattergram between observed and simulated daily sediment yield (Calibration years 2003) - Simulated sediment (t/ha) 2.5 2 1.5 1 r 2 = 0.9307 0.5 0 0 1 2 3 Observed sediment (t/ha)
400 350 Rainfall Observed Simulated 0 100 Runoff (mm) 300 250 200 150 100 50 200 300 400 500 600 700 800 Rainfall (mm) 0 6 7 8 9 10 6 7 8 9 10 2002 2003 Months 900 Graphical comparison of observed and simulated monthly runoff (Calibration years 2002-2003)
Results of statistical analysis for model calibration Monthly (2002-2003) Statistics Runoff (mm) Sediment yield (t/ha) Observed Simulated Observed Simulated Mean 105.34 115.66 3.51 3.58 Standard deviation 115.67 120.10 3.07 3.64 Maximum peak 326.28 353.9 9.07 11.82 Total 1053.88 1156.61 35.02 35.82 Count 10 10 10 10 t-calculated -1.344 0.069 t-critical (two tailed) 2.262 2.262 r 2 0.959 0.938 % deviation -9.800-2.267
Regression line 1:1 line 400 350 y = 1.017x + 8.5341 r 2 = 0.9594 Simulated runoff (mm) 300 250 200 150 100 50 0 0 50 100 150 200 250 300 350 400 Observed runoff (mm) Scattergram between observed and simulated monthly runoff (Calibration years 2002-2003)
12 Rainfall Observed Simulated 0 Sediment yield (t/ha) 11 10 9 8 7 6 5 4 3 2 1 100 200 300 400 500 600 700 800 Rainfall (mm) Graphical comparison of observed and simulated monthly sediment yields (Calibration years 2002-2003) 0 900 6 7 8 9 10 6 7 8 9 10 2002 2003 Months Scattergram between observed and simulated monthly sediment yield (Calibration years 2002-2003) Simulated sediment yield (t/ha) 12 11 10 9 8 7 6 5 4 3 2 Regression line y = 1.219x - 0.6876 r 2 = 0.9384 1:1line 1 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Observed sediment yield (t/ha)
Results of statistical analysis for model validation Daily (2004) Statistics Runoff (mm) Sediment yield (t/ha) Observed Simulated Observed Simulated Mean 2.34 2.51 0.077 0.082 Standard deviation 11.20 13.58 0.319 0.446 Maximum peak 120.7 154.2 3.38 5.12 Total 357.7 384.09 11.79 12.65 Count 153 153 153 153 t-calculated -0.698-0.453 t-critical (two tailed) 1.975 1.975 r 2 0.976 0.949 % deviation -7.378-7.294 COE 0.926 0.935
Results of statistical analysis for model validation Monthly (2004-2005) Statistics Runoff (mm) Sediment yield (t/ha) Observed Simulated Observed Simulated Mean 124.08 130.49 3.64 4.35 Standard deviation 103.31 109.61 2.81 3.29 Maximum peak 320.70 359.78 8.92 11.12 Total 1240.84 1304.98 36.41 43.5 Count 10 10 10 10 t-calculated -0.789-2.153 t-critical (two tailed) 2.262 2.262 r 2 0.946 0.910 % deviation -5.16-19.46
Results of sensitivity analysis of calibrated SWAT model Parameters Values Annual runoff % deviation Annual sediment % deviation (mm) from base (t/ha) from base Overland 'n' 0.06 462.60-0.01 3.99 0.25 0.12 463.33 0.15 3.66-8.04 Channel 'n' 0.025 462.20-0.10 4.51 13.32 0.065 463.79 0.25 3.78-5.03 FFC* 0.50 472.49 2.13 4.47 13.74 1.00 492.71 6.50 4.40 11.96 Base variable Calibrated 462.65 --- 3.98 ---
Statistical analysis of the observed and simulated nutrient losses (2003-2005) Statistics Organic N Organic P NO 3 -N Soluble P Obs. Sim. Obs. Sim. Obs. Sim. Obs. Sim. Mean 0.374 0.327 0.185 0.207 0.160 0.141 0.015 0.013 Standard deviation 0.430 0.411 0.344 0.458 0.227 0.167 0.013 0.011 Maximum 1.540 1.500 2.120 2.950 1.550 1.280 0.050 0.040 Total 26.185 22.91 12.931 14.476 11.227 9.846 1.051 0.891 Count 70 70 70 70 70 70 70 70 t-calculated 1.655 0.391 1.470 1.145 t-critical (two tailed) 2.201 2.201 2.201 2.201 r 2 0.92 0.95 0.83 0.80 % deviation 12.5-11.9 12.3 15.3
Results of statistical analysis for model validation for daily rainfall generation Statistical parameters Mean (mm) Standard deviation Maximum (mm) Total (mm) Count t-calculated t-critical (two-tail) 1.96 r 2 0.98 % deviation Chhokranala watershed (2001-2005) Observed 3.17 11.47 173.4 5783.6 1826 10.52 14.5 Simulated 2.71 10.06 162.8 4942.9 1826
Results of statistical analysis for model validation for monthly rainfall generation (2002-2005) Statistical parameters Mean (mm) Standard deviation Maximum (mm) Total (mm) Count t-calculated t-critical (two-tail) 2.012 r 2 0.82 % deviation Chhokranala watershed Observed 96.80 145.54 543.70 4646.40 48 Simulated 1.546 14.1 83.13 133.15 465.00 3990.41 48
Statistical analysis for the monthly observed and simulated rainfall, runoff and sediment yield during the monsoon period Statistical parameters Rainfall (mm) Runoff (mm) Sediment (t/ha) Observed Simulated Observed Simulated Observed Simulated Chhokranala watershed Mean 216.0 194.40 119.82 118.36 3.571 Standard deviation 161.34 146.44 105.32 107.39 2.871 Maximum 543.70 465.0 326.28 353.9 9.07 Total 4320.0 3888.08 2396.35 2367.26 71.43 Count 20 20 20 20 20 t-calculated 1.033 0.152 0.888 t-critical (two-tail) 2.093 2.093 2.093 r 2 0.67 0.85 0.86 % deviation 10.0 1.2 6.7 3.331 3.205 10.81 66.63 20
Identification of critical sub-watersheds Sub- Watershed Area (km 2 ) Runoff (mm) Sediment (t/ha) Organic N (kg/ha) Organic P (kg/ha) NO 3 -N (kg/ha) Soluble P (kg/ha) Erosion class Priority WS1 185.2 502.0 8.49 5.39 2.96 3.75 0.05 M - WS2 290.8 491.1 9.44 5.83 3.26 3.72 0.05 M - WS3 119.4 521.7 6.43 4.19 2.28 3.94 0.05 H - WS4 316.8 444.2 8.76 6.36 3.34 3.24 0.05 H - WS5 278.7 662.5 21.02 10.70 6.26 4.78 0.06 V. H. I WS6 280.4 519.8 15.86 9.42 5.17 3.91 0.05 H II WS7 259.6 490.7 13.01 7.71 4.33 3.71 0.05 H III WS 1730.9 518.9 11.86 7.09 3.94 3.86 0.05 - - Soil erosion classes Slight Moderate High Very high Severe Very severe Soil erosion range 0-5 5-10 10-20 20-40 40-80 >80
Tillage treatments Tillage treatments Code Mixing efficiency Zero tillage T1 0.05 Conservation tillage T2 0.25 Field cultivator T3 0.30 M. B. plough T4 0.90 Conventional tillage T5 0.50 Fertilizer levels for different crops Fertilizer level (code) Rice Maize G-nut Soybean Existing (F1) 25:15 (5:5) 1/2 of the recommended (F2) 40:30 (10:10) Recommended (F3) 80:60 (20:20) 20:15 (5:5) 50:30 (10:5) 100:60 (20:10) 10:20 (5:5) 20:40 (10:5) 30:60 (15:10) 10:20 (5:5) 30:30 (10:5) 60:60 (15:10)
WS5 of Chhokranala Watershed Crop Runoff (mm) Sediment (t/ha) Organic N (kg/ha) Organic P (kg/ha) NO 3 -N (kg/ha) Soluble P (kg/ha) Grain yield (t/ha) Rice 666.5 21.016 10.698 6.264 4.780 0.062 0.601 G-nut 657.5 24.769 11.461 6.647 5.859 0.076 1.127 Maize 661.3 28.858 12.791 7.353 4.660 0.078 0.480 Soybean 662.4 24.230 11.259 6.544 6.194 0.087 0.838
Results of Management for WS5 (Chhokranala Watershed) Treatments Runoff (mm) Sediment (t/ha) NO 3 -N (kg/ha) Soluble P (kg/ha) Organic N (kg/ha) Organic P (kg/ha) T1+F1 662.70 07.74 4.67 0.06 03.85 02.06 T2+F1 662.62 13.60 4.76 0.06 07.23 04.15 T3+F1 662.49 15.52 4.77 0.07 07.29 03.89 T4+F1 661.34 28.86 4.66 0.08 12.79 07.35 T5+F1 662.49 21.02 4.78 0.06 10.70 06.26 T1+F2 662.79 07.71 5.31 0.06 06.07 03.90 T2+F2 662.77 13.42 5.41 0.06 11.67 07.84 T3+F2 662.73 15.19 5.43 0.07 11.28 07.23 T4+F2 662.65 26.34 5.46 0.10 13.01 07.15 T5+F2 662.75 20.78 5.44 0.06 17.51 11.93 T1+F3 663.34 07.22 6.62 0.06 09.63 06.92 T2+F3 663.28 12.75 6.72 0.06 19.68 14.55 T3+F3 663.39 14.34 6.78 0.07 18.51 13.31 T4+F3 663.22 26.17 6.81 0.10 20.74 13.56 T5+F3 663.22 20.36 6.76 0.06 30.85 23.01
Conclusions 1. Manning's 'n' values for overland flow and channel flow are 0.040 and 0.025, respectively for the Chhokranala watershed. 2. The SWAT model accurately simulates monthly runoff and sediment yield from the Chhokranala watershed. 3. The SWAT model accurately simulates nutrient losses from the Chhokranala watershed on daily basis. 4. The weather generator can be used to simulate monthly rainfall and thereby runoff and sediment yield. The model can be used for planning and management of the small agricultural watersheds on long-term basis using generated daily rainfall.
5. The SWAT model can successfully be used for identifying critical sub-watersheds for management purpose. 6. The sub-watershed WS5 and WS6 and WS7 of Chhokranala watershed found to be critical. 7. Crops like maize, groundnut and soybean can not replace the existing rice crop, on the basis of sediment and nutrient losses reduction criteria. 8. Zero tillage, conservation tillage and field cultivator along with 40:30 kg/ha of N:P can be recommended because these tillage practices reduce sediment yield as compared to existing tillage and nutrient losses being within the permissible limit.